It is well known to use prosthetic joint replacements in patients with various kinds of disorders affecting the joints, including degenerative disorders, such as severe osteoarthritis.
Over the years; a vast array of materials have been developed and utilised in the construction and manufacture of such prostheses. This is partly because the knowledge base regarding materials, and relevantly biocompatible materials, has been growing. It is also because, despite technological advances, there are a continuing number of complications associated with joint replacement prostheses with which surgeons and patients must grapple. As a result, surgeons and other inventors in the field have had, and are still challenged with, an ongoing quest to improve on the ease of insertion of the prostheses, to reduce the incidence of long and short term complications associated with using them, and to improve on the longevity of both the bio-prosthetic interface and the prostheses themselves.
Certain metal alloys, such as CrCoMo alloy, are widely used for the bearing surfaces of prosthetic joints in humans. Attractive features of such alloys include their biocompatibility, their relatively high wear resistance, and their relative ease of manufacture through casting and machining. Wrought CrCoMo alloy bar is also used, especially for femoral heads.
Following casting, the CrCoMo alloy is often Hot Isostatic Pressed (HIP) to reduce grain size and reduce the internal porosities. These porosities are exposed during machining and create holes on the surface. During cooling and the formation of the grains, carbides such as silicon carbide and molybdenum carbide, can, however, precipitate at the grain boundaries and within the grains. These carbides are extremely hard compared to the surrounding CrCoMo alloy and after machining and polishing of the articulating surface usually stand above the surface. While the carbides only stand above the surface by a relatively small distance, their presence serve to increase the wear of complementary surfaces adapted to engage the CrCoMo surface.
In addition, the process of machining and polishing the CrCoMo alloy components exposes the parts to a variety of abrasive and potentially toxic compounds and particles. While every attempt is made to remove these foreign particles, an implant made of inherently porous material is difficult to clean perfectly. Studies of the surface of commercially available implants have shown there to be residual scratches, aluminum oxide abrasive particles and other foreign matter in an apparently clean, polished component. FIG. 1 is a scanning electron micrograph (SEM) of such a surface revealing the scratches, pores and other foreign matter on a surface that had been polished and cleaned using known standard techniques.
Polishing is also a problem as it can alter the dimensional accuracy of the component. While the life of a bearing surface of a component may be modelled by finite element analysis (FEA), it is generally assumed that the component is “as designed”. Studies of commercially available parts that have been hand polished show, however, great variation in the radii of curves on the bearing surfaces. Variations greater than 20% have been noted in some cases. Such variation can lead to the implant not performing in the manner expected of it from FEA.
Coated implants, such as ceramic coatings and PVD Nitride coatings, have also been tried in a bid to reduce the wear of the material surface, such as the polyethylene component, that is adapted to articulate with the CrCoMo alloy surface in an implant. These coatings have generally failed due to inadequate adhesion in the highly aggressive environment of joint replacement. The presence of carbides in the implant may also adversely affect the process of coating wherein the coating does not adhere to the carbides or other contaminants on the implant. This leads to areas of weakness in the coating and therefore adversely affects the bond strength of the coating. In addition, coating a roughened surface often only serves to harden the roughened details.
The present invention is directed to a new method of preparing a prosthetic that preferably does not suffer the problems of the prior art.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed in Australia before the priority date of each claim of this application.